Sialic acid (1) ##STR2## exists at a terminal of glycoproteins and glycolipids, and is involved in physiological activities of glycoproteins and glycolipids. Sialic acid derivatives are actively synthesized so as to clarify functions of sialic acid at a molecular level.
In particular, research and development of sialidase inhibitors are actively conducted as an agent for prophylaxis and therapy of influenza virus. Consequently, as effective sialidase inhibitor in vitro and in vivo, 4-amino- and 4-guanidino-2-deoxy-2,3-didehydrosialic acids (2 and 3) ##STR3## are reported (M. von Itzstein et al.: Nature, 363, 418-423 (1993); M. von Itzstein et al.: Carbohydrate Research, 259, 301-305 (1994); JP-A-5-507068). It is said that no drug-resistant virus will appear at first with respect to the derivatives. However, development of highly active compounds without cross-resistance is desired, since it is well predictable that drug-resistant viruses will appear clinically.
In addition, 7-fluoro-2,7-dideoxy-2,3-didehydrosialic acid is reported (international publication WO95/32955). However, sialidase inhibiting activities thereof are not disclosed.
7-fluorosialic acid derivatives of said compound whose hydroxyl group at 4 position is substituted has not been synthesized and will be useful as a drug, since the substitution is thought to affect physiological activities thereof.
Objects of the invention relate to 4-substituted-2,7-dideoxy-7-fluoro-2,3-didehydro-sialic acids which are expected as drugs such as an antiviral agent, an immunomodulating agent and an antitumor agent, synthetic intermediates thereof and use application thereof.